An IMS/MMD (IP Multimedia Subsystem/Multimedia Domain) network or architecture primarily comprises several signaling entities such as proxy-call session control function (P-CSCF), interrogating-CSCF (I-CSCF), serving-CSCF (S-CSCF), and home subscriber service (HSS) which is usually a database or other repository for user or subscriber information such as authorization data, including information related to services provided to a user. Roaming service and mobility are supported by a combination of Session Initiation Protocol (SIP) components such as the signaling entities, P-CSCF, S-CSCF, I-CSCF, and mobile IP components or nodes, such as home agent (HA) and foreign agent (FA). IMS/MMD architecture mandates that there should be security association (SA) between the mobile and P-CSCF. Secure Internet Protocol (IPSec) is one way of providing SA for signaling and media traffic.
In IMS, authentication of a user, or user's equipment (UE), can be achieved using authentication and key agreement (AKA). Authentication is achieved between the UE, generally a mobile, and its home network even though the SIP messaging is transported over the Serving, or visiting, network. This allows home network-based control of access to IMS resources, while the visited network controls bearer resources over the packet data servicing nodes (PDSN). SIP Registration and Response messages are used to transport the IMS/AKA protocol payloads. These messages are sent from the UE to the S-CSCF and vice versa. The S-CSCF queries the HSS to obtain security related parameters for the UE. IMS AKA uses a challenge response mechanism to authenticate the UE to the Home Network. The UE uses a long term key to compute a response to a challenge sent by the S-CSCF via the P-CSCF. The P-CSCF plays no role in challenge generation aside from acting as a forwarding element.
In an IMS/MMD network, the signaling and media travel to their destination, such as to S-CSCF, via the HA which usually resides on the home network. This long route or path from a Mobile Node or correspondent node (CN) to a destination through the home network is a phenomenon called trombone routing. Because trombone routing impacts both registration and call setup methods, trombone routing hinders efficiency during a mobile's boot-strapping (registration, re-registration, call setup) in the visited network and during its movement from one subnet to another. This causes both an initial call setup delay, and a handoff delay when the mobile moves from one visited subnet to another. In addition, during a mobile's movement between subnets, AKA is performed as part of registration; hence, a faster registration will help establish an expedited SA, thus reducing the handoff delay.
FIG. 1 shows an example of the inefficiency associated with the trombone routing in the Mobile IPv4 (MIPv4) foreign agent-care-of address (FA-CoA) case. Here, even if the P-CSCF is situated in the same visited network as the mobile node (MN), the signaling related to registration has to traverse all the way to HA in the home network before getting routed to P-CSCF. This inefficiency is partly due to the reverse tunneling associated with the FA-CoA case. Similarly, any incoming call or INVITE signaling message from a CN traverses, via P-CSCF, to HA in the home network before being delivered to the Mobile Node in the visited network. This traversal increases the call setup delay. Since registration is delayed due to trombone routing, the handoff is also delayed as the mobile moves to a new network and sets up a new SA.
Hence, as shown in FIG. 1a), the path of a SIP registration message with trombone routing in MIPv4 FA-CoA is:
MN→FA1→HA→P-CSCF→S-CSCF
and the path of a SIP registration Reply message is:
S-CSCF→P-CSCF→HA→FA1→MN.
Similarly, the path of a SIP INVITE, as shown in FIG. 1b), is:
CN→S-CSCF→P-CSCF→HA→FA1→MN
and the path of a SIP OK is:
MN→FA1→HA→P-CSCF→S-CSCF→CN
FIG. 2 shows trombone routing in the Mobile IPv6 (MIPv6) case, and illustrates how trombone routing affects the efficiency when MIPv6 is used. Unlike the MIPv4 case, MIPv6 does not use FA. While using MIPv6, it is customary to use the Mobile Node's home address in the contact field during the Session Initiation Protocol (SIP) registration and re-registration process even if Mobile Node obtains a new CoA from the access router during each handoff. Thus, during the re-registration process, a new P-CSCF's address is provided to the HSS, while the contact address of the Mobile Node remains same. HA, of course, keeps a mapping of Mobile Node's home address and its most recent CoA by means of MIP registration.
Since there is no FA in the visited network in MIPv6, the mobile obtains the new CoA using stateless auto-configuration. When a mobile registers with S-CSCF in the home network, the mobile provides its home address as its contact address. Since there is a reverse tunneling between the mobile and HA, both the call setup and registration (re-registration) process are subjected to trombone routing.
As shown in FIG. 2a), the path of a SIP registration message with trombone routing in MIPv6 is:
MN→HA→P-CSCF→S-CSCF
and the path of a SIP registration Reply message is:
S-CSCF→P-CSCF→HA→MN.
Similarly, the path of a SIP INVITE, as shown in FIG. 2b), is:
CN→S-CSCF→P-CSCF→HA→MN
and the path of a SIP OK is:
MN→HA→P-CSCF→S-CSCF→CN
Thus, just like the case of MIPv4, the trombone routing will affect the performance. As is evident from both of these cases, trombone routing is undesirable.
Similarly, there is an inherent trombone routing problem with data or media, because the reverse tunneling is used by default. FIG. 3 shows trombone routing associated with media delivery for both the MIPv6 without route optimization, and the MIPv4 FA CoA-based approach. In MIPv4, MIP data is tunneled between Visited 1 and Home, and then the data travels, non-tunneled, from Home to CN. In MIPv6, the data travels from MN, in Visited 1, to CN in Visited 2, through HA, Home, so that the data passes through the home network when traveling from visited Network 1 to visited Network 2. Although reverse tunneling can offer advantages, this trombone routing contributes to the handoff delay because it necessitates traversing a long path via the home network.
FIG. 4 illustrates another affect of trombone routing in MIPv4. When an IMS mobile node, MN, moves from network A to network B as shown in FIG. 4, the MIP registration and SIP re-registration must be completed at network B as follows. First, Mobile Node detects its mobility through the FA advertisement from the FA at network B. Once Mobile Node detects the mobility, it invokes a MIP registration through the FA and dynamic host configuration protocol (DHCP)-client operation to get the internet protocol (IP) address of the new P-CSCF at network B. At this point, the routing table of the Mobile Node has been updated through the MIP operation, and the tunnel between the FA and HA has been established, so that the Mobile Node can be reachable from any node in the network. After getting the IP address of the P-CSCF from the DHCP server at network B, the Mobile Node invokes a SIP re-registration by sending a SIP registration message to the new P-CSCF. The P-CSCF forwards the SIP message to the S-CSCF that, in turn, replies back to the P-CSCF with a SIP response message. Accordingly, the Mobile Node receives the SIP response message and the SIP re-registration is completed.
In this handoff process, there are two issues. The first is slow handoff. As shown schematically in FIG. 5, the sequential operations of FA advertisement detection, MIP registration, DHCP, and SIP registration increase the handoff delay.
The second issue is inefficient routing. Because of the reverse mode of tunneling between the FA and HA, the SIP messages between the Mobile Node and P-CSCF take the trombone routing path. Hence, as shown in FIG. 6, the path of a SIP message from the Mobile Node to the P-CSCF is:
MN→FA→Gateway in Network B→Gateway in Home Network→HA→Gateway in Home Network→Gateway in Network B→P-CSCF
A SIP message from the P-CSCF to the Mobile Node takes the reverse path.
Thus, trombone routing causes inefficiencies and delays in both registration and handoff.
The following abbreviations are used throughout.    AAA: authentication, authorization and accounting    AKA: authentication and key agreement    BSC: base station controller    BTS: base transceiver station    CDMA: code division multiple access    CN: correspondent node    CoA: care-of Address    DHA: dynamic home agent (aka mobility agent MA)    DHCP: dynamic host configuration protocol    DNS: domain name service    ESP: encapsulating security payload    FA: foreign agent    HA: home agent    HAA: Home-Agent-MIP-Answer    HAR: Home-Agent-MIP-Request    HHA: handover answer    HHR: handover request    HSS: home subscriber service    IMS: IP Multimedia Subsystem    IMS/MMD—combination of IMS and MMD    IPSec: suite of security protocols    MAC: message authentication code    MIPv4—Mobile IPv4    MIPv6—Mobile IPv6    MMD—Multimedia Domain    MN: mobile node    MPA: media independent pre-authentication    NAI: Network Access Identifier    PCF: packet control function    P-CSCF—Proxy Call Session Control Function    PDSN—Packet Data Serving Node    PPP: point to point protocol    RAN: radio access network    RTP: real-time transport protocol    SA: security association    S-CSCF—Serving Call Session Control Function    SIP: session initiation protocol    SRTP: secure real-time transport protocol    UE: user equipment    URI: Universal Resource Identifier